In recent years, since a so-called lithium ion secondary battery in which a carbon material such as graphite or the like is used in a negative electrode and a lithium-containing metal oxide such as LiCoO2 or the like is used in a positive electrode is high in the capacity, it is put into practical use, as a potential electric storage device, as a main power supply mainly of a note personal computer and a portable telephone. A lithium ion secondary battery is a so-called rocking chair type battery where a battery is, after assembling, charged to supply lithium ions from a lithium-containing metal oxide of a positive electrode to a negative electrode and, during discharge, lithium ions at a negative electrode are returned to a positive electrode, and is characterized in having high voltage and high capacitance.
On the other hand, under the circumstance where an environmental problem is highlighted, an electric storage device (main power source and auxiliary power source) for use in an electric automobile or a hybrid automobile that will replace a gasoline engine is under active development. Furthermore, as an electric storage device for use in an automobile, a lead battery has been used. However, since a vehicular electric equipment and instrument have been improved, from the view points of energy density and output density, a novel electric storage device is in demand.
As such a novel electric storage device, the lithium ion secondary battery and an electric double layer capacitor are gathering attention. However, while the lithium ion secondary battery is high in the energy density, there remain problems in the output characteristics, safeness and cycle lifetime. On the other hand, an electric double layer capacitor, although it is used as a memory backup power source of an IC and LSI, is smaller in the discharge capacity per one charge than a battery. However, the electric double layer capacitor is excellent in the instantaneous charge and discharge characteristics and has such high output characteristics as that charges and discharges of several tens thousands cycles or more are withstood, which a lithium ion secondary battery lacks, and the maintenance free property.
An electric double layer capacitor has such advantages as mentioned above. However, the energy density of an existing general electric double layer capacitor is substantially 3˜4 Wh/l, that is, substantially two digits smaller than that of a lithium ion secondary battery. When an electric automobile is considered, it is said that the energy density of 6˜10 Wh/l is necessary for putting into practical use and 20 Wh/l is necessary for commercializing.
As an electric storage device compatible to applications where such high energy density and high output characteristics are necessary, recently, an electric storage device called as a hybrid capacitor in which electricity storage principles of a lithium ion secondary battery and an electric double layer capacitor are combined is gathering attention. In a hybrid capacitor, usually, a polarizable electrode is used in a positive electrode and a non-polarizable electrode is used in a negative electrode. The hybrid capacitor is gathering attention as an electric storage device that combines high energy density of a battery and high output characteristics of an electric double layer. On the other hand, in the hybrid capacitor, a capacitor in which a negative electrode capable of storing and releasing lithium ions is brought into contact with metal lithium to let chemically or electrochemically store and carry (hereinafter, also referred to as dope) lithium ions in advance to lower a negative electrode potential, thereby the withstand voltage is heightened and the energy density is made considerably larger is proposed. (Patent documents 1 through 4)
In the hybrid capacitor, although high performance can be expected, when lithium ions are doped in a negative electrode, metal lithium has to be adhered to over an entire negative electrode. Furthermore, although it is possible as well to locally dispose metal lithium partially in a cell to bring into contact with a negative electrode, there are problems in that doping takes a very long time and uniform doping over an entire negative electrode is difficult. In particular, a large high capacity cell such as a cylindrical electric storage device where electrodes are wound or a rectangular electric storage device where a plurality of sheet electrodes is laminated is said difficult to put into practical use.
However, the problem came to a settlement at one stroke by an invention where, when holes that penetrate through both sides of a negative electrode current collector and a positive electrode current collector that constitute a cell are disposed and, through the holes, lithium ions are transported and simultaneously metal lithium that is a lithium ion supply source and a negative electrode are short-circuited, only by disposing metal lithium at an end portion of a cell, lithium ions can be doped over an entire negative electrode in a cell (patent document 5). Lithium ions are doped usually in a negative electrode. However, it is disclosed in patent document 5 that, similarly to the above, the lithium ions can be doped, together with in a negative electrode, in a positive electrode or, in place of in the negative electrode, in a positive electrode.
Thus, even in a large cell such as a cylindrical electric storage device where electrodes are wound or a rectangular electric storage device where a plurality of sheet electrodes is laminated, lithium ions can be doped in a short time to an entire negative electrode in the device and uniformly over an entire negative electrode, thereby the withstand voltage is improved to result in drastically increasing the energy density. As the result, a forecast that a capacitor having high capacitance together with large output density that an electric double layer capacitor intrinsically has is realized is obtained.
However, in order to put such a high capacitance capacitor into practical use, higher withstand voltage, higher capacitance, higher energy density and lower internal resistance are in demand.
Patent document 1: JP-A-08-107048
Patent document 2: JP-A-09-055342
Patent document 3: JP-A-09-232190
Patent document 4: JP-A-11-297578
Patent document 5: WO98/033227